Heat assembly and method of transferring heat

ABSTRACT

A heat sink assembly includes a printed wiring board, a metal case and a circuit package containing a gallium arsenide field effect transistor heat dissipating circuit. The circuit package includes a metal slug formed integrally with the circuit package, the heat dissipating circuit being bonded to an obverse surface of the metal slug. The printed wiring board includes first and second metal lands, the first metal land being disposed on an obverse surface of the printed wiring board, the second metal land being disposed on a reverse surface of the printed wiring board. A solder film is formed bonded to and thermally coupling a reverse surface of the metal slug to the first metal land, and a plurality of solder posts are formed, each post bonding to and thermally coupling the first metal land to the second metal land. The metal case is pressed against the second metal land with a grease film of thermally conductive grease squeezed therebetween. At least one bolt extends through a hole in the printed wiring board and into the metal case so as to squeeze together the metal case, the printed wiring board, the first and second metal lands and the grease film.

This application is a division of application Ser. No. 08/705,609, filedAug. 30, 1996 now U.S. Pat. No. 5,739,586.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a heat sink assembly for a cabletelevision line amplifier. In particular, the invention relates to aheat sink assembly to transfer heat from a GaAs FET amplifier integratedcircuit to a metal case housing a printed wiring board on which theamplifier integrated circuit is mounted.

2. Description Of Related Art

Heat dissipating surface mount technology has been described. Forexample, see "A New Surface Mount Power Package" by Alexander Ehnert andJean-Luc Diot, published in the Proceedings of I.E.E.E. Applied PowerElectronics Conference, APEC '93, conference date Mar. 7-11, 1993,incorporated herein by reference. However, Ehnert and Diot describeusing copper filled through holes, and do not describe using solderposts formed in a reflow solder operation. Furthermore, Ehnert and Diotdo not disclose mounting a gallium arsenide integrated circuit amplifierfor wide band usage in the package. This is significant in that wideband gallium arsenide integrated circuit amplifiers as described hereindissipate more heat than do comparable sized power semiconductors, thusrequiring improved thermal conduction to a cold sink.

U.S. Pat. No. 4,942,076 to Panicker, et al. describes a ceramicsubstrate with metal filled via holes for hybrid microcircuits. Panickeret al describe a high frequency gallium arsenide die mounted on aceramic substrate with via holes that use metal filings to carry itsinternally generated heat through the ceramic to a heat sink.

However, Panicker et al. do not describe a microcircuit package with aheat slug bonded to a land of a printed wiring board.

SUMMARY OF THE INVENTION

It is an object to the present invention to provide for efficient heattransfer from a gallium arsenide field effect transistor amplifierintegrated circuit to a casing housing a printed wiring board on whichis mounted the integrated circuit amplifier. It is a further object ofthe present invention to provide this heat transfer assembly usinginexpensive mass production technology. It is yet another object toprovide thermal coupling between the integrated circuit amplifierpackage and the printed wiring board using reflow solder processingtechniques.

These and other objects are achieved in a heat sink assembly thatincludes a printed wiring board, a metal case and a circuit packagecontaining a gallium arsenide field effect transistor heat dissipatingcircuit. The circuit package includes a metal slug formed integrallywith the circuit package, the heat dissipating circuit being bonded toan obverse surface of the metal slug. The printed wiring board includesfirst and second metal lands, the first metal land being disposed on anobverse surface of the printed wiring board, the second metal land beingdisposed on a reverse surface of the printed wiring board. A solder filmis formed bonded to and thermally coupling a reverse surface of themetal slug to the first metal land, and a plurality of solder posts areformed, each post bonding to and thermally coupling the first metal landto the second metal land. The metal case is pressed against the secondmetal land with a grease film of thermally conductive grease squeezedtherebetween. At least one bolt extends through a hole in the printedwiring board and into the metal case so as to squeeze together the metalcase, the printed wiring board, the first and second metal lands and thegrease film.

BRIEF DESCRIPTION OF DRAWINGS

The invention will be described in detail in the following descriptionof preferred embodiments with reference to the following figureswherein:

FIG. 1 is a plan view of a top of a heat sink assembly;

FIG. 2 is a section view of a circuit package and circuit board of theheat sink assembly;

FIG. 3 is a section view of the completed heat sink assembly; and

FIG. 4 is a detail section view of a portion of the section of FIG. 3.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Improvements in line amplifiers used in cable television distributionsystems have enabled wide band operation to provide subscribers with anincreased number of channels. To achieve high performance, wide bandoperation, gallium arsenide field effect transistor integrated circuitamplifiers are being used. See U.S. patent application Ser. No.08/686,022, "FIELD EFFECT TRANSISTOR CABLE TELEVISION LINE AMPLIFIER",filed Jul. 25, 1996, incorporating herein by reference. These wide bandamplifiers suffer from greater heat dissipation in the amplifier circuititself than more narrow band counterparts. When greater heat dissipationis encountered with conventional heat sink assemblies, the activetransistors are forced to operate at elevated temperatures. Temperatureelevation of an active transistor by as much as 20° C. has been shown toreduce the life of the transistor by half. In order to achieve a longerlife amplifier, it is necessary to improve the heat transfer assembly.

In FIG. 1, circuit package 10 is mounted on circuit board 20. Circuitboard 20 may be a multi layer printed wiring board or a two-sided wiringboard. Such printed wiring boards are formed of a substrate that hascopper cladding on at least their obverse and reverse surfaces. Thecopper cladding is etched to leave the circuit wiring desired, groundplanes, and in the present application, thermally conductive lands 22. Ahole for a bolt is drilled in the printed wiring board at 34. The boardsare prepared for component mounting, for example, by tin plating.

FIG. 2 is a section view of section 2--2 of FIG. 1. In FIG. 2, circuitpackage 10 includes heat dissipating circuit 12, for example, a highheat dissipation gallium arsenide field effect amplifier integratedcircuit. Circuit package 10 further includes heat slug 14. Preferablyheat slug 14 is formed from tin plated copper. Integrated circuitamplifier 12 is eutecticly bonded to heat slug 14 through eutectic bond16. Preferably eutectic bond 16 employs a brazing (or soldering) alloythat melts at a temperature sufficiently above the temperature of leadtin solder that subsequent soldering operations do not remelt bond 16.Case 18 encloses amplifier integrated circuit 12 and heat slug 14 and isformed of materials selected for other properties (for example, plasticselected for a hermetic seal). Case 18 is preferably formed of plastic.Leads 8 extend out of circuit package 10. Circuit board 20 includessubstrate 30 preferably formed of epoxy-fiberglass or FR4. Substrate 30has disposed thereon lands 22 on opposite surfaces of substrate 30. Pads24 are also formed on a surface of substrate 30. Pads 24 and lands 22are formed out of a copper sheet (e.g., 0.001 or 0.002 inches of copper)clad to substrate 30 by patterning and etching.

In the present invention, via holes 26 are drilled through lands 22 andsubstrate 30. Via holes 26 are preferably plated through holes.Subsequently, tape 32 (preferably Kaptont™ tape) is applied to land 22on the reverse side of substrate to hold solder in via hole 26 duringsubsequent reflow and wave solder operations.

Solder paste 28 is screened (e.g., like silk screening) onto land 22 andpads 24 on the obverse side of substrate 30 to a prescribed thicknessusing a solder paste stenciling machine. The solder paste is preferablya paste comprised of 10% rosin flux and 90% finely powdered solder. Thesolder is preferably a eutectic blend composed of 62% tin, 36% lead, and2% silver.

Circuit package 10 is then mounted on circuit board 20, preferably usinga surface mount placement machine.

Circuit board 20 with mounted circuit package 10 is then passed throughan infra red heat reflow oven, typically 215° C. for 60 seconds. In thisoven, the rosin and solder of the solder paste melt and flow. Solderpaste 28 over pads 24 becomes solder that bond leads 8 to correspondingpads 24. Solder paste 28 over land 22 becomes solder that bonds heatslug 14 to land 22, and furthermore, flows solder into via holes 26 toform solder posts bonded to and thermally coupling land 22 on theobverse surface of substrate 30 to land 22 on the reverse surface ofsubstrate 30.

The printed wiring board may also mount components that are through-holemounted. In this case, the components are mounted, either by hand or bymachine, and the assembled printed wiring board is wave soldered in awave soldering machine, typically 260° C. for 10 seconds.

Next, tape 32 is removed.

FIG. 3 depicts the improved heat sink assembly with circuit board 20(and circuit package 10 mounted thereon) installed in metal case 50.Metal case 50 is preferably a complete case enveloping circuit board 20.Shown in FIG. 3, is a portion of metal case 50 having a protrusion 52.Protrusion 52 is preferably flat so as to efficiently and thermallyconfront land 22 on the reverse side of substrate 30. The assembly isformed by applying thermally conductive grease 54 onto the flat portionof protrusion 52. Thermally conductive grease 54 is preferably a silicongrease with a high concentration of zinc (or beryllium) oxide dispersedtherein. Circuit board 20 is installed in case 50 so that land 22 on thereverse surface of substrate 30 squeezes the thermally conductive grease54. Bolts 56 are installed through bolt holes 34 (FIG. 1) in circuitboard 20 and into the protrusion portion 52 of metal case 50. As thebolts are tightened, excess grease is squeezed out of the gap betweenprotrusion portion 52 and land 22 on the reverse side of substrate 30.Preferably, the grease is thermally squeezed out except for voids andpores in the materials so that protrusion 52 directly contacts land 22in much of the area.

Metal case 50 is preferably formed from diecast aluminum using 380alloy.

Studies have been conducted to determine the optimal size of via holes,the hole-to-hole spacing in the grid of via holes, the thickness andarea of solder paste 28 (FIG. 2) so as to define total solder volume,the area of and dimensions of heat slug 14, and the area and dimensionsof lands 22. It is desired to completely fill the via holes with asolder post, and at the same time bond the heat slug 14 to the land 22with a thermally conductive solder film, and at the same time bond oneend of solder post 44 to land 22 on the reverse side of substrate 30,and at the same time bond the solder post to the plated metal sleeve inthe via hole.

Heat slug 14 preferably measures 0.138 inches by 0.344 inches. Thelarger the heat slug, the greater is the difficulty of manufacturingcircuit package 10. For example, a large heat slug may tend to developcracks in the body of circuit package 10 when processed to produce theheat sink assembly described herein. On the other hand, a large heatsink is desired to provide a good thermal coupling between heat slug 14and land 22 on the obverse surface of substrate 30.

The quality of the thermal coupling and bond between heat slug 14 andland 22 on the obverse surface of substrate 30 is adversely effected bya rough surface on the bottom of heat slug 14. The bottom of heat slug14 should be co-planar to within 0.007 inches, preferably within 0.004inches.

The process of stenciling deposits solder paste onto lands 22 and pads24. The thickness of the solder pastes should be between 7.32 and 15.29thousandths of an inch, preferably between 10 and 12 thousandths of aninch. Preferably, this thickness is defined by an optimal thickness forbonding lead 8 to pad 24 (and other bonded leads).

However, sufficient solder is needed to flow into and fill via holes 26.To achieve sufficient solder, the area of land 22 on the obverse surfaceof substrate 30 is made larger than the area of heat slug 14, preferably40% larger in area. The opening in the stencil through which solderpaste 28 is deposited is preferably adjusted so that the area of solderpaste 28 over land 22 on the obverse surface of substrate 30 is about50% larger than the area of heat slug 14.

Each via hole is preferably a plated through hole. During the process offabricating the printed wiring board, copper plating forms a coppersleeve inside the hole. Preferably the whole copper clad board is tinplated to avoid oxidation of copper.

A via hole may be considered "filled" if upon examination there is atleast 75% of the hole's cross section area filled with solder. Thesolder flows into the plated through hole so as to form a superiorthermal path from the bottom surface of heat slug 14 through the solderpost and into the plated copper sleeve inside the plated through hole.Heat flows through both the solder post and the plated copper sleeveinto land 22 on the reverse surface of substrate 30. Heat in the platedcopper sleeve efficiently flows to land 22 on the reverse side ofsubstrate 30. The solder posts as described herein bond to the platedcopper sleeve so that as much as 63% of the heat from slug 14 flowsthrough the plated copper sleeve, and only 37% of the heat must passthrough the solder post, to reach land 22 on the reverse surface ofsubstrate 30.

In the area of heat slug 14 (that measures 0.138 inches by 0.344inches), there are preferably 28 via holes evenly distributed (e.g.,about 0.040 inches on centers). The via holes should be between 0.012and 0.022 inches in diameter, preferably 0.016 inches in diameter.

Solder paste 28 first begins to melt and fill the via holes in thereflow oven and then completes filling the via holes when the printedwiring board is processed through the wave solder machine (in which theprinted wiring board with components thereon is "floated" on a pool ofmolten solder). This process uses standard reflow solder steps and wavesolder steps (i.e., process steps optimized for surface mount ofcomponents in the reflow oven and through hole mount of components inthe wave solder process). Special adjustments are not needed to affectthis heat sink.

The solder posts formed in the via holes are bonded to the bottomsurface of heat slug 14. A measure of this bond is the shear forcerequired to break off circuit package 10. A good solder joint would havea theoretical optimum shear strength of about 8,000 pounds per squareinch. The shear strength of the solder joints in the grid of solderposts should average to be about half of this theoretical optimum shearstrength (e.g., 4,000 pounds per square inch). Tests were performed, andit was confirmed that the posts achieved shear strength over 4,000pounds per square inch. In some tests, the fiberglass in substrate 30broke before the bond between solder posts and heat slug 14, and inother tests, the casing around circuit package 10 cracked before thebond between solder posts and heat slug 14.

FIG. 4 is a detailed section view corresponding to FIG. 3. In FIG. 4,lands 22 on the obverse and reverse sides of substrate 30 are connectedwith plated metal sleeves 22A. Plated metal sleeves 22A are preferablyformed as plated through via holes when substrate 30 and lands 22 areformed. Plated metal sleeves 22A are preferably formed of copper metaland then protected from oxidation with a thin tin plating. The processof forming plated metal sleeves 22A is preferably carried out usingknown plated through hole techniques compatible with plated through holerequirements of remaining portions of the printed wiring board.

The thickness and area of solder paste 28 defines the volume of solderof the solder body. The solder body forms thin solder film 42A and aplurality of solder posts represented as 42B, 42C. Solder film 42A bondsmetal slug 14 to metal land 22 on the obverse surface of substrate 30,and conducts heat from metal slug 14 to metal land 22. Solder post 42Bpreferably fills plated metal sleeve 22A so that solder post 42B extendsto and bonds with metal land 22 on the reverse surface of substrate 30,and solder post 42B conducts heat from metal slug 14 to metal land onthe reverse surface of substrate 30.

On occasion, it may happen that a solder post such as solder post 42Cdoes not completely fill metal sleeve 22A. However, even in such asituation, solder post 42C bonds with metal sleeve 22A. Solder post 42Cconducts heat from metal slug 14 to metal sleeve 22A, and metal sleeve22A, in turn, conducts heat to metal land 22 on the reverse surface ofsubstrate 30. The void in metal sleeve 22A left by the incompletelyformed solder post 42C is filled by a small portion 54A of thermalgrease 54.

With this arrangement, a direct heat conductive path extends from heatdissipating circuit 12 through eutectic bond 16, through metal slug 14,through solder body having film 42A and posts 42B, 42C, through metallands 22 and plated metal sleeves 22A, through thermal conductive grease54 and into metal case or chassis 50. Bolts 56 (FIG. 3) squeeze most ofgrease 54, except for void that are filled, from between land 22 andmetal case 50. Thus, the thermal path from heat dissipating circuit 12to metal case 50 conducts heat through metals, not ceramics, notfiberglass, etc. Thus, improved heat transfer performance is achievedusing conventional reflow and wave soldering operations. When metal case50 is maintained at 100° C. and heat dissipating circuit 12 dissipates6.2 W in an environmental air temperature of 60° C., the heat slugtemperature rises no more than 22° C., typically 9° C.

Having described preferred embodiments of a novel heat sink assembly andmethod of transferring heat (which are intended to be illustrative andnot limiting), it is noted that modifications and variations can be madeby persons skilled in the art in light of the above teachings. It istherefore to be understood that changes may be made in the particularembodiments of the invention disclosed which are within the scope andspirit of the invention as defined by the appended claims.

Having thus described the invention with the details and particularityrequired by the patent laws, what is claimed and desired protected byletters patent is set forth in the appended claims:

What is claimed is:
 1. A method of transferring heat from a heatdissipating circuit comprising steps of:conducting heat from the heatdissipating circuit to a metal slug through a eutectic bond; conductingheat from the metal slug through a solder body having a plurality ofsolder posts into a corresponding plurality metal sleeves disposed in acorresponding plurality of via holes through a substrate; and conductingheat from the plurality of metal sleeves into a metal land clad to areverse surface of the substrate.
 2. The method of claim 1, furthercomprising a step of conducting heat from the metal land through athermally conductive grease to a metal case.
 3. The method of claim 1,further comprising a step of further conducting heat from the metal slugthrough the solder posts into the metal land.
 4. The method of claim 1,further comprising steps of:further conducting heat from the metal slugthrough a solder film of the solder body into another metal land clad toan obverse surface of the substrate; and conducting heat from saidanother metal land through a bolt into a metal case.
 5. A method offorming a heat transfer assembly comprising steps of:forming a printedwiring board to include first and second metal lands and a plurality ofvia holes extending through the first and second metal lands; applying apiece of tape to the second metal land on a reverse surface of theprinted wiring board so as to cover the plurality of via holes; applyinga solder paste over the first land on an obverse surface of the printedwiring board so as to cover the plurality of via holes; mounting acircuit package over the solder paste over the first metal land on theobverse surface of the printed wiring board, the circuit packageincluding a metal slug integrally formed with the circuit package andincluding a heat dissipating circuit eutectically bonding the heatdissipating circuit to the metal slug; and heating the printed wiringboard with mounted circuit package in a reflow solder oven so as to forma bonding solder film between the metal slug and the first land and soas to form a plurality of solder posts in the plurality of via holesbonded to and thermally coupling the first metal land to the secondmetal land.
 6. The method of claim 5, further comprising stepsof:removing the tape; applying a thermally conductive grease to thesecond metal land; assembling the printed wiring board in a metal case;and installing a fastener through the printed wiring board, through thefirst and second metal lands and into the metal case so as to squeezethe thermally conductive grease between the second metal land and themetal case.
 7. The method of claim 1, wherein the solder body includes asolder film, the method comprising a further step of conducting heatfrom the metal slug through the solder film to another metal land cladto an obverse surface of the substrate.
 8. The method of claim 7,further comprising a step of conducting heat from the metal land on thereverse surface of the substrate through a thermal greese film into ametal case.
 9. The method of claim 7, further comprising a step ofconducting heat from said another metal land on the obverse surface ofthe substrate through a fastener film into a metal case.
 10. The methodof claim 9, further comprising a step of conducting heat from the metalland on the reverse surface of the substrate through a thermal greesefilm into the metal case.
 11. The method of claim 1, wherein the solderbody includes a solder film, the method comprising further stepsof:conducting heat from the metal slug through the solder film toanother metal land clad to an obverse surface of the substrate; andconducting heat from the metal slug through at least one solder postdirectly into the metal land on the reverse surface of the substrate.12. The method of claim 11, further comprising a step of conducting heatfrom the metal land on the reverse surface of the substrate through athermal greese film into a metal case.
 13. The method of claim 11,further comprising a step of conducting heat from said another metalland on the obverse surface of the substrate through a fastener filminto a metal case.
 14. The method of claim 13, further comprising a stepof conducting heat from the metal land on the reverse surface of thesubstrate through a thermal greese film into the metal case.
 15. Amethod of transferring heat from a heat dissipating circuit comprisingsteps of:conducting heat from the heat dissipating circuit to a metalslug through a eutectic bond; conducting heat from the metal slugthrough a solder body having a plurality of solder posts disposed in acorresponding plurality of via holes through a substrate; and conductingheat from at least one of the solder posts into a metal land clad to areverse surface of the substrate.
 16. The method of claim 15, furthercomprising a step of conducting heat from the metal land on the reversesurface of the substrate through a thermal greese film into a metalcase.
 17. The method of claim 15, wherein a plurality metal sleevesintegrally formed the metal land are disposed in the plurality of viaholes, the method further comprising steps of:conducting heat from atleast one of the solder posts into corresponding ones of the pluralityof metal sleeves; and conducting heat from the corresponding metalsleeves into the metal land.
 18. The method of claim 17, furthercomprising a step of conducting heat from the metal land on the reversesurface of the substrate through a thermal greese film into a metalcase.
 19. The method of claim 15, wherein the solder body includes asolder film, the method comprising a further step of conducting heatfrom the metal slug through the solder film to another metal land cladto an obverse surface of the substrate.
 20. The method of claim 19,further comprising a step of conducting heat from the metal land on thereverse surface of the substrate through a thermal greese film into ametal case.
 21. The method of claim 19, further comprising a step ofconducting heat from said another metal land on the obverse surface ofthe substrate through a fastener film into a metal case.
 22. The methodof claim 19, further comprising a step of conducting heat from the metalland on the reverse surface of the substrate through a thermal greesefilm into the metal case.